Oh no! Another at8rc collimation thread!

Ok so tonight I was checking my new at8rc for collimation and I discovered the the light baffle that is attached to the primary cell has a nosepiece that can be unscrewed. This makes adjusting the primary way simpler. Essentially I followed the instructions on the RCOS website. Did anyone know about this?

I just checked this on my AT8RC and it unscrews all the way down at the primary. (the whole baffle would come out) I only turned it, didn't remove it. It was indeed only finger tight, But are you talking about removing the entire baffle tube? A quick picture would help if you have a chance. Anything to make collimation a little easier would be great.

On my model (an Altair Astro badged version bought about 3 years ago), the primary baffle is also finger tight and the whole baffle unscrews. However this does not loosen the mirror as the mirror is held in place by a collar that the baffle screws into. There's some pics that hopefully explain better on these pages:

I do however recall reading some threads about GSO supplying an extension to the primary baffle that was designed to stop stray light getting into the primary baffle from around the sides of the secondary. This extension simply screwed into the end of the original primary baffle. Perhaps the OP's scope has this extension fitted and that is what he is unscrewing?

I completely removed the primary baffle. When looking down the focuser with a collimating cap, I was able to clearly see the gap shown in fig.7 of the RCOS instructions. This gap must be used for primary collimation and it is otherwise blocked by the long primary baffle.

Thanks for you description of collimating the AT8RC. I haven't gotten mine dialed in perfect yet, but I did get the site cap you suggested in another thread, and it is much easier to use than the Celestron cheshire I was using. I need some clear sky for a star test to see how close I am. Depending on that I'll try removing the baffle.

Thanks, my primary was off, I couldn't see any of the ring of light with the cheshire, but the star test showed something was off as I couldn't get things centered just by adjusting the secondary. I was able to see a very very thin ring of light with the site cap, and adjusted the primary to center it. Only a star test will tell. Thanks again.

You will find countless threads on this topic. What worked best for me was to remove the primary baffle then follow the instructions on the RCOS website. I used a refractor cheshire to align the secondary on the center spot, however if you have a Takahashi collimating scope that would be even more accurate.

After a very minor tweak with a star test I was able to see a nice airy disk.

I have an update on my 8"RC collimation that I would like to share with others.

Recently my scope developed an odd behavior. All of a sudden the stars looked really bad at the center of the frame which means the primary is off. The night before I got excellent stars, the following night it was a mess.

Something obviously had shifted, I tried collimating again but the star test did not agree with my images.

So I decided to take the rear cell apart and the culprit became very apparent, I was able to rock the mirror by a small amount. Not huge but surely enough to throw off my images.

I removed the mirror, to make sure that nothing under it was off, I re-set the gasket that is between the mirror and its holder, then replaced the mirror holding ring. This needs to be firmly screwed in place, finger tight is good. It can't be too tight as you don't want to pinch the optics. This ring also has two set screws to lock it into place.

After resetting the mirror I quickly visually aligned by re-insalling the primary cell minus the mirror light baffle. This makes visual alignment of the primary a breeze.

Then I put the light baffle in place and fine tuned the secondary alignment using its center spot and my trusty sight cap.

A quick check of the reflection with the light baffle back in place confirmed that the light baffle - and the mirror - were very well aligned. This means that the mirror and focuser are also reasonably aligned.

The star test was excellent requiring no further tweaking and my test subs confirmed that I was very well collimated.

Another interesting fact is that the primary-secondary spacing needs to be accurate. When I started my first image my stars looked like little triangles and my focus point had shifted in by about 1,500 microns. The secondary needed to be moved closer the primary. A couple of trials followed by a test shot put me back to within 150 microns of the original focus point (when I bought the scope). The stars were back to be nice and round with an even slight elongation at the four corners. The spacing may be further refined with a Ronchi eyepiece.

So my conclusions are that the scope can not be precisely collimated unless the mirror and the focuser are square to each other. The very simple assembly tho make this relatively easy. I suspect that a lot of these scopes have loose mirrors that shift during the course of the night. A lot of people blame this on the fact that the focuser is held by the mirror holder but I think that the loose mirror may be a more common cause. This is of course just speculation at this point.

I thought that this information may be useful to others who are scratching their heads over collimation of these RC's.

I own an AT12RC that I bought used in April of this year. I don't know if my experience with collimation will be of any benefit. I have used a lot of collimation tools. What I found most effective is a 2" autocollimator with my 12" RC. Using the 2" autocollimator I would find that after collimation that the secondary vane reflections would not be in alignment with the secondary vanes. They were offset by quite a bit. When I used a laser collimator, I would find that the returning red LED spot would not return to the original LED laser spot but was offset a little. My images with a 6D camera had very badly distorted star images in the corners of the FOV. I ordered the focuser tilt plate accessory and it allowed me to better collimate my 12" RC. The center of the secondary in my 12" RC is not marked, so I used a LaserMax holographic collimator to adjust the focuser tilt plate to center the Laser collimation pattern on the secondary mirror, and that corrected my collimation and corner star issues.

Thanks Gary!
The autocollimator is a very precise way to collimate these RC's. I actually bought a Takahashi collimating scope as it's easy to use and very accurate.

I don't think that my scope needs the focuser tilt plate. I made sure that the mirror was snug on the holder and tightened down the mirror retaining ring and the visual test confirms that everything is reasonably nice and square.

My scope appears to be very well collimated at this point as I was able to split the double double and the star test looks very nice and symmetric. It's also worth noting that I get very little elongation at the corners without any corrector. So far this 8" looks like a great deal:)

Good to hear of your success in collimating your 8" RC. I hope I never need to remove the primary mirror but if I do, this thread will be a big help. In 2011, I imaged all year with an AT6RC and it took me three days and a lot of advice from friends to get it collimated. It held collimation well when hauling it to the field. With the AT12RC when I was using APS-C (1.6 crop factor) cameras, corner stars were okay. But when I switched to a 6D full frame camera, corner stars got pretty ugly. I was able to buy the focuser tilt plate from Teleskop -Express in Germany. This is what they have on their website about the device:

"Regarding the focuser's adjustment, we're talking about minimal amounts of tipping. If using a DSLR camera with APS-C sensor (or other CCD cameras up to approx. 23mm sensor diagonal), you won't have any problems with field definition. But if you have a full-frame camera, correcting the focuser can increase off-axis image sharpness."

I see that the tilt plate is also available for the AT6RC and AT8RC, but I definitely didn't need one for my AT6RC.